Support varying numbers of residuals in autodiff.

This commit modifies the only function in autodiff that takes a
templated number of outputs (i.e. residuals) and makes that
template parameter a normal parameter. With that change, it
is a trivial matter to support a dynamic number of residuals.

The API for dynamic residuals is to pass a fake number of
residuals as the second template argument to
AutoDiffCostFunction, and to pass the real number of
parameters as a second constructor argument.